Energetics

Question 1

What does enthalpy change mean

Answer 1

Change in energy under constant pressure

Question 2

What is the standard state for pressure

Answer 2

100 kPa

Question 3

What is the standard temperature

Answer 3

298 K

Question 4

How do you work out enthalpy change

Answer 4

Reactants minus products

Question 5

Can you draw an energy profile for an endothermic reaction

Answer 5

Insert pic

Enthalpy change is usually positive
Products have more energy than the reactants

Question 6

Can you draw a reaction profile for an exothermic reaction

Answer 6

Insert pic

Enthalpy change is usually negative
And reactants have more energy than products
Example of exothermic: combustion

Question 7

What does exothermic mean

Answer 7

To give out heat to the surroundings

Question 8

What does endothermic mean

Answer 8

To take in heat energy from surroundings

Question 9

State what is meant by the term enthalpy of combustion

Answer 9

the enthalpy change required to burn one mole of substance completely under oxygen under standard states and conditions

Question 10

State what is meant by the term enthalpy of formation

Answer 10

the enthalpy change required to form one mole of a substance from its constituent atoms in its standard state under standard conditions

Question 11

what is the equation for energy change?

Answer 11

Q = mc∆T

Question 12

what errors may someone make in a calorimetry experiment

Answer 12

we may ignore that energy absorbed by equipment

the reaction may be too slow or incomplete

Question 13

what is the equation to work out the enthalpy of combustion

Answer 13

enthalpy of combustion = reactants minus products

Question 14

what is the equation to work out the enthalpy of formation

Answer 14

enthalpy of formation = products- reactants

Question 15

State what is meant by the term Hess’s law

Answer 15

the enthalpy change of a reaction is independent of the route taken

Question 16

state what is meant by the term mean bond enthalpy

Answer 16

the energy required to break the covalent bond into gaseous atoms averaged over different molecules

Question 17

why do we use Hess’s law to work out enthalpy change of a reaction

Answer 17

sometimes it’s hard to measure enthalpy change for some reactions. It can be hard to add the exact amount of a product so it’s done in excess

Question 18

How is energy lost in the calorimetry experiment

Answer 18

• heat loss surroundings

- absorbed heat goes to tripod